FIG. 1 shows a heated enclosure system where many and preferably all the components of a sampling system are heated and maintained at a temperature above the dew point of the hydrocarbon gas being sampled.
When sampling period ends and it is time to collect the filled bottle and replace it with an empty bottle at a given sampling station, an operational problem occurs that has not been resolved by the prior art.
The typical procedure for commissioning an empty bottle at a sampling station is to connect the bottle to the sampling system and then, since the bottle contains air, to perform a purging process to purge air out of the sample line from the pump to the bottle, from the bottle connection, and from the bottle itself. This must be done because air will contaminate the sample and make the BTU analysis inaccurate.
The purging process consisting of several purge cycles is typically performed to purge the sampling system of air. A purge valve 5 on the sample pump is opened to allow sample gas to flow freely in response to pipeline pressure around the pump to the “sample out” line 4. With the purge valve 5 open, the operator performs the first purge cycle. Valve 1 is first opened to pressurize fresh bottle 2 to pipeline pressure. Valve 3 is then closed again, valve 1 is opened, and this purge cycle is repeated, typically about three or more times for higher pipeline pressures, and up to ten times for lower pipeline pressures. Finally, the purge valve is closed, the bottle is bled down to near zero, and outlet valve 3 is closed again to complete the purging process.
With the purging process complete, the operator begins the sampling period during which a number of gas samples are taken from the pipeline and added to the sample bottle. The sampling period generally lasts about a month, during which the accumulated samples may pressurize the bottle to nearly pipeline pressure.
A typical prior art bottle connection system is shown in FIG. 2. Sample out line 4 connects to and supplies gas for gas sampling to distribution block 7, which is fastened to the top inside surface of the heated enclosure as shown in FIG. 1. Gauge 8 is typically found in the system to monitor pressure in the sample line. Male quick connector member 9 is permanently threaded into block 7. Hand operated fastening nut 10 engages the female threads of the bottle quick connection member 11 to attach the bottle assembly to the gas distribution block with O-ring 17 perfecting the seal.
Member 11 is permanently attached to inlet valve 1 as the bottle side component of the hand operated quick connect system. An alternate method of connecting the bottle 2 to the distribution block 7 would be to use a male/male outlet valve 1 and thread valve 1 directly into block 7. This is not preferred due to the continued space and the time involved in making that type of connection. Bleed body 12 which carries bleed stem 13 is typically closed and is opened prior to disconnect by turning handle 15 to allow trapped high pressure gas to vent from cavity 18, allowing hand operated disconnect of the bottle using connection nut 10. With systems of this or similar type, there is no way to purge all the air from cavity 18 after connection of the new sample bottle.
A major problem with prior art is that the purging process artificially increases the percentage of “heavy” rich gas components present in the sample. If the purging process is performed on a bottle that is “cold,” i.e., below the dew point of the gas being sampled, some of the heavy components of the gas will condense on the inner walls of the sample bottle during each purging cycle. Because the purging process requires several purging cycles, the sample bottle will become “loaded” with heavy gas components accumulated during each purging cycle. When that bottle is taken to a lab for analysis, it is heated to revaporize any components in the gas that had condensed out of the vapor phase during the trip to the lab, so that when the gas vapor is run through the chromatograph it will contain all of the constituents of the gas in the sample bottle. This heating will revaporize heavy components accumulated during the purging process and cause the gas analysis to be incorrectly high in BTU content.
Another problem with prior art is the need to perform multiple purge cycles to effectively purge the sampling system. This multiplies the number of steps the operator must perform, which, in turn, multiplies the chance of operator error, as well as the time and expense involved in the purging process.
The disadvantages of the prior art are overcome by the present invention, and an improved purging system for use in a heated enclosure housing a sampling system along a gas pipeline is hereinafter disclosed. The invention also includes the use of a new purge block for use when replacing a sample bottle.